Process for urea production involving a carbon dioxide stripping step

ABSTRACT

An improved process for urea production as well as a method of retrofitting a pre-existing urea plant based on the Stamicarbon process are disclosed. According to the invention, a high-yield reactor with partial removal of the reaction heat and a urea recovery section of the solution leaving the high-yield reactor, are added to the pre-existing urea plant, means being provided for recycling unreacted ammonia and carbon dioxide vapors as well as a carbamate solution obtained in the urea recovery section to the pre-existing reactor.

This is a divisional of application Ser. No. 08/178,749 filed Jan. 7,1994, now abandoned.

DESCRIPTION

1. Field of Application

The present invention relates to an improved process for ureaproduction, as well as to a method for retrofitting pre-existing ureaproduction plants in which a reaction mixture leaving an urea synthesisreactor is submitted to a stripping step with carbon dioxide(Stamicarbon process).

The invention also relates to a plant for implementing the abovementioned process.

2. Background Art

As is well known, the need often arise of increasing the urea productioncapacity of a pre-existing plant.

To this end, various methods of enhancing the production capacity havebeen proposed heretofore, such as that described in European Patentapplication EP-A-0 479 103 by the same Applicant.

EP-A-0 479 103 discloses a process wherein highly pure ammonia andcarbon dioxide are reacted in a first high-yield "once-through" reactor,the reaction mixture thus obtained is fed to a recovery section and asynthesis reaction between less pure reagents, substantially recycledfrom the recovery section (recovery mixture), is carried out in thepre-existing reactor of conventional type.

In copending U.S. application Ser. No. 08/059,241 by the same Applicant,the problem of increasing the urea production capacity and reducingenergy consumptions in pre-existing plants according to the StamicarbonProcess (STC), i.e. including a CO₂ stripping section, is addressed bycarrying out urea synthesis in the first high-yield reactor in adiabaticconditions (in a so-called "Vulcan" reactor) and by distilling thereaction mixture leaving the Vulcan reactor so as to obtain unreactedammonia and carbamate streams recycled respectively to the Vulcanreactor itself and to the pre-existing low-yield reactor.

The process thus achieved appeared to-date to be satisfactory andfurther improvements did not seem to be possible.

Continuing his research and experiments, however, the Applicant was ableto develop a new perfected revamping process applicable to the existingplants based on the total recycle process, with the aim of notablyincreasing the urea production capacity and reducing the energyconsumption.

The aim of the present invention, therefore, is that of providing animproved process of producing urea which allows to obtain highconversion yields, a reduction of investment and operation costs, betterplant exploitation and more ample overload margins of the existingequipment, and considerable plant debottlenecking.

SUMMARY OF THE INVENTION

This aim is accomplished, according to the invention, by a process ofproducing urea comprising the steps of:

reacting ammonia and carbon dioxide in a first reaction space;

effecting a gas stripping with carbon dioxide of a first reactionmixture leaving said first reaction space, so as to obtain a purifiedurea solution and unreacted ammonia and carbon dioxide vapors;

feeding said purified urea solution to a first urea recovery section;

condensing at a predetermined pressure said unreacted ammonia and carbondioxide vapors in a carbamate condenser;

reacting ammonia and carbon dioxide in a second reaction space;

wherein the method further comprises the steps of:

a) submitting a second reaction mixture leaving said second reactionspace to a first carbamate decomposition treatment at a pressuresubstantially equal to the pressure of the carbamate condenser, so as toseparate a first stream of unreacted ammonia and carbon dioxide vaporsfrom a liquid stream including urea;

b) recycling the first stream of unreacted ammonia and carbon dioxidevapors to said carbamate condenser;

c) submitting the liquid stream including urea to a second carbamatedecomposition treatment so as to separate a second stream of unreactedammonia and carbon dioxide vapors from a purified urea solution;

d) condensing said second stream of unreacted ammonia and carbon dioxidevapors and recycling the condensate thus obtained to said carbamatecondenser.

Urea synthesis in the first reaction space may be carried out at processconditions currently used in STC plants, such as--for example--atpressures of from 130 to 200 bar and temperatures of from 180 to 200° C.

The carbamate condensing step, the CO₂ -stripping step of the reactionmixture leaving said reaction space and the urea recovery step in saidrecovery section, may also be carried out according to the known processconditions used in a STC plant.

Preferably, the urea synthesis in the second reaction space is carriedout in a high-yield "once through" reactor, at pressures of from 240 to260 bar and at temperatures of from 190 to 200° C.

A "once through" reactor with partial reaction heat removal has beenobserved to be particularly suitable for revamping urea productionplants involving a CO₂ stripping step.

Most preferably, the "once through" reactor comprises two parts: aprimary section with reaction heat removal and a secondary section ofconventional type.

For the purposes of the present invention, particular advantages interms of energy recovery are achieved when the "primary reactor" is ofthe so-called "Kettle" type.

According to the invention, the reaction mixture leaving the high-yieldreaction space is submitted to a carbamate decomposition treatment so asto separate unreacted ammonia and carbon dioxide vapors from a liquidstream including urea.

More particularly, this reaction mixture is submitted to two carbamatedecomposition treatments in series.

The first carbamate decomposition treatment is carried out in a highpressure decomposer at a pressure substantially equal to the pressureexisting in a carbamate condenser provided upstream of the firstreaction space.

Preferably, such a pressure ranges from from 140 to 150 bar.

Most advantageously, a first stream of unreacted ammonia and carbondioxide vapors is obtained which may be directly recycled to theaforementioned carbamate condenser and further reacted in the firstreaction space.

The second carbamate decomposition treatment of the solution leaving thefirst carbamate decomposer is carried out in a second carbamatedecomposer at a pressure of from 6 to 18 bar, most preferably from 8 to14 bar.

In this way, a second stream of unreacted ammonia and carbon dioxidevapors is obtained which is first condensed with the aid of an aqueouscarbamate solution leaving the urea recovery section, and then recycledto the carbamate condenser as well.

The second carbamate decomposition treatment also yields a purified ureasolution which may be either fed to a low-pressure distiller of the urearecovery section, or collected in a tank downstream of the low-pressuredistiller before the finishing treatments in a vacuum section.

Advantageously, the carbon dioxide/urea molar ratio in the strippingtreatment may be adjusted within optimal values by feeding up to 20% ofthe reaction mixture flow leaving the first urea synthesis reactor tothe first carbamate decomposition treatment.

Preferably, from 10 to 15% of the reaction mixture flow leaving thefirst urea synthesis reactor by-passes the the CO₂ stripping treatmentand is directly fed to the first carbamate decomposition treatment.

According to a further aspect of the present invention, it has beenfound after intensive studies and research that it is surprisinglypossible to enhance in a simple and safe way the production capacity ofa pre-existing urea production plant including a CO₂ stripping step.

When a capacity increase of over 20% of a pre-existing plant isrequired, it is advantageously possible to apply the concept of HighEfficiency Parallel Reactor in order to accomplish the desired object ofachieving a reduction of investment and operation costs, better plantexploitation and more ample overload margins of the existing equipment,and considerable plant debottlenecking.

The application of such concept to a pre-existing plant for ureaproduction including:

a urea synthesis reactor;

a carbamate condenser upstream of said urea synthesis reactor;

a carbon dioxide stripper downstream of said urea synthesis reactor;

means for feeding a first reaction mixture leaving said first reactor tothe carbon dioxide stripper;

a urea recovery section for separating urea from the first reactionmixture leaving the carbon dioxide stripper;

is carried out, in accordance with the invention, by a method ofretrofitting comprising the steps of:

a) providing a second urea synthesis reactor connected with means forfeeding high purity ammonia and carbon dioxide;

b) providing a second urea recovery section including at least a firstand a second carbamate decomposers in series downstream of said secondurea synthesis reactor;

c) providing conduit means for recycling unreacted ammonia and carbondioxide vapors leaving the top of said first carbamate decomposer tosaid carbamate condenser;

d) providing means for condensing unreacted ammonia and carbon dioxidevapors leaving the top of said second carbamate decomposer;

e) providing means for recycling the condensate thus obtained to saidcarbamate condenser.

More particularly, of from 30 to 50% of the new requested productioncapacity is obtained in an additional high yield reactor (75%) of the"once through" type without recycle.

Most preferably, said reactor operates with partial removal of thereaction heat and comprises two parts: a primary section with reactionheat removal and a secondary section of conventional type.

Further aspects and advantages of the invention will be better apparentfrom the following description of preferred, though non-limitative,embodiments thereof carried out hereinbelow referring to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a schematic layout of a urea production plant including astripping step with CO₂ according to the prior art;

FIG. 2 shows a schematic layout of the urea production plant of FIG. 1retrofitted in accordance with a first embodiment of the presentinvention;

FIG. 3 shows a schematic layout of the urea production plant of FIG. 1retrofitted in accordance with a second embodiment of the presentinvention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The urea production plant shown in FIG. 1 comprises a urea synthesisreactor 201-D to which pure ammonia (NH₃) and carbon dioxide (CO₂) arefed by conventional compressor means 104-J/JS and 102-J.

More particularly, ammonia is first fed to a carbamate condenser 202-Cby conduit means 117, while carbon dioxide is fed to a stripper 201-C byconduit means 107 and then to the carbamate condenser 202-C by conduitmeans 201 before entering the urea synthesis reactor 201-D.

The stripper 201-C is also fed by the reaction mixture leaving the ureasynthesis reactor 201-D through conduit 207.

With 104 is indicated a conventional urea recovery section, comprising alow-pressure distiller 301-C with relevant separator 301-E, two vacuumevaporators 401-C and 402-C with relevant separators 401-F and 402-F, aswell as an ammonia vacuum concentration section 50.

The features of the urea recovery section 104 are per se conventionaland well known in the art and will not be further described in detail.

In operation, a large part of the carbamate and part of the ammoniacontained in the urea solution leaving the reactor 201-D are stripped in201-C and recycled to the reactor, while a urea solution leaving thestripper 201-C is obtained having a relatively low CO₂ (7÷9% weight) andNH₃ (5÷8% weight) residues.

This solution is treated in the urea recovery section 104 where it isdistilled at 3÷4 bar abs in 301-C: the vapors thus obtained are sent tocondenser 302-C yielding a carbamate solution which is recycled to theurea synthesis reactor 201-D by means of pump 301-J/JS.

The main technical characteristics of the isobaric stripping processwith CO₂ can be summarized as follows:

synthesis pressure: about 140÷145 bar abs

NH3/CO2 mol in the reactor: about 2.8÷3.0

H2O/CO2 mol in the reactor: about 0.4÷0.5

temperature of the reactor: about 185° C.

yield: about 57÷58%

steam consumption: about 900÷1000 kg/MT urea

According to a first embodiment of the present invention shown in FIG.2, a high efficiency reactor ROT and a urea recovery section DIST forpurifying a urea solution leaving the latter, are connected in parallelto the preexisting urea synthesis reactor 201-D.

The reactor ROT is fed by highly pure carbon dioxide by means ofcompressor means KA-1 and highly pure ammonia by pump means PA-1A/1S andis connected to the urea recovery section DIST through conduit 108.

Advantageously, the reactor ROT is of the "once-through" high yield typewith partial removal of the reaction heat.

The "once through" reactor ROT comprises two parts: a primary sectionR-1 of the so-called "Kettle" type with reaction heat removal and asecondary section R-2 of conventional type.

The urea recovery section DIST preferably comprises at least twocarbamate decomposers E-1 and E-2 connected in series through conduitmeans 110.

Most advantageously, the first carbamate decomposer E-1 operates at apressure substantially equal to that of the existing stripper 201-C,while the second carbamate decomposer E-2 operates at approximately 10bar abs.

According to an aspect of the present invention, a first stream leavingcarbamate decomposer E-1, including unreacted ammonia and carbon dioxidevapors, is directly recycled to the carbamate condenser 202-C, operatingat the same pressure, through conduit 111.

A second stream including unreacted ammonia and carbon dioxide vapors,which leaves the top of carbamate decomposer E-2, is first condensed inE-3 and then recycled to the carbamate condenser 202-C through conduitmeans 112 and pump 301-J/JS.

From the bottom of carbamate decomposer E-2 a purified urea solution isobtained, which may be either fed through conduit 113 to thelow-pressure distiller 301-C of the urea recovery section 104, ordirectly collected to storage tank 302-F trough conduit M.

In the first instance, the urea solution leaving the carbamatedecomposer E-2 is further treated together with the urea solutionleaving the stripper 201-C in the low-pressure distiller 301-C.

According to a second embodiment of the present invention shown in FIG.3, the pre-existing urea synthesis reactor 201-D is connected, throughby-pass line BP to urea recovery section DIST' which in this caseincludes two carbamate decomposers E'-1, E'-2 designed so as towithstand the new load.

In this embodiment as well, the purified urea solution leaving columnE'-2 may be either fed to the low-pressure distiller 301-C, or by-passthe same and and be directly collected into storage tank 302-F.

Further features and advantages of the invention, will be apparent fromthe non-limitative examples given hereinbelow, wherein revamping of anexisting 1500 MT urea production plant with CO₂ stripping isillustrated.

EXAMPLE 1

The aim of the revamping is to increase the production capacity of apre-existing plant from 1500 MTD to 2250 MTD, by adding a new reactor ofthe "once through" type in parallel to the existing one.

In the case herein considered, the optimal capacity distribution is asfollows:

    ______________________________________                                        existing reactor:                                                                            1500 MTD urea                                                  new reactor:    750 MTD urea                                                  Total:         2250 MTD urea                                                  ______________________________________                                    

The operating conditions of the reactors are:

a) Reference Case

Existing reactor (201-D):

Capacity: 1500 MTD

NH3/CO2 mol: 2.85

H2O/CO2 mol: 0.40

Yield : 57%

P: 140 bar abs

T: 183° C.

b) New Conditions

b.1 Existing reactor (201-D):

Capacity: 1500 MTD

NH3/CO2 mol: 3.0

H2O/CO2 mol: 0.53

Yield: 57%

P: 145 bar abs

T: 185° C.

b.2 New reactor (ROT):

Capacity: 750 MTD

NH3/CO2 mol: 3.6

H2O/CO2 mol: 0

Yield: 75%

P: 242 bar abs

T: 193° C.

The average weighed yield of the two reactors operating in parallel is:##EQU1## i.e. 6 points percent more than the reference case.

This corresponds to a lower specific steam consumption of 120÷150 kg/MTurea, despite the capacity increase.

EXAMPLE 2

In Example 1, the capacity of the existing reactor in the new operatingconditions was 1500 MTD of urea, i.e. equal to the design.

The existing stripper in this case will have at disposal for the ureasolution stripping a lower quantity of CO₂ gas than the design (about83% of the design), the balance will be as carbamate solution.

This lower quantity of CO₂ is anyway sufficient to achieve a goodperformance in the stripper 201-C.

In any case, it is possible to approach or substaantially reach thedesign carbon dioxide/urea ratio in the stripper 201-C, by operatingaccording to the embodiment shown in FIG. 3.

The mentioned design ratio is achieved in this case by feeding about 17%of the reaction mixture flow leaving the pre-existing reactor 201-D tothe carbamate decomposer E'-1 by-passing the stripper 201-C.

This aliqout of the reaction mixture is sent, through line BP, to thecarbamate decomposer E'-1 of the new urea recovery section DIST', forthe carbamate decomposition treatment.

Thanks to this by-pass, all the existing synthesis, stripping and urearecovery sections will work very close to or substantially within designconditions, avoiding any critical overload of these sections.

I claim:
 1. A process of producing urea comprising the steps of:reactingammonia and carbon dioxide in a first reaction space; effecting a gasstripping with carbon dioxide of a first reaction mixture leaving saidfirst reaction space, so as to obtain a purified urea solution andunreacted ammonia and carbon dioxide vapors; feeding said purified ureasolution to a first urea recovery section; condensing at a predeterminedpressure said unreacted ammonia and carbon dioxide vapors in a carbamatecondenser; reacting ammonia and carbon dioxide in a second reactionspace;wherein the method further comprises the steps of: a) submitting asecond reaction mixture leaving said second reaction space to a firstcarbamate decomposition treatment at a pressure substantially equal tothe pressure of the carbamate condenser, so as to separate a firststream of unreacted ammonia and carbon dioxide vapors from a liquidstream including urea; b) recycling the first stream of unreactedammonia and carbon dioxide vapors to said carbamate condenser; c)submitting the liquid stream including urea to a second carbamatedecomposition treatment so as to separate a second stream of unreactedammonia and carbon dioxide vapors from a purified urea solution; d)condensing said second stream of unreacted ammonia and carbon dioxidevapors and recycling the condensate thus obtained to said carbamatecondenser.
 2. A process according to claim 1, wherein said firstcarbamate decomposition treatment is carried out at a pressure of from140 to 150 bar.
 3. A process according to claim 1, wherein said secondcarbamate decomposition treatment is carried out at a pressure of from 6to 18 bar.
 4. A process according to claim 1, wherein urea synthesis inthe second reaction space is carried out at pressures of from 240 to 260bar and at temperatures of from 190 to 200° C. by means of a high-yield"once-through" reactor with partial removal of the reaction heat.
 5. Aprocess according to claim 1, further comprising the step of feeding thepurified urea solution obtained from said second carbamate decompositiontreatment to a low-pressure distiller of said urea recovery section. 6.A process according to claim 5, wherein the purified urea solutionobtained from said second carbamate decomposition treatment is feddownstream of a low-pressure distiller of said urea recovery section. 7.A process according to claim 1, further comprising the step ofsubjecting part of the first reaction mixture leaving said firstreaction space to said first and second carbamate decompositiontreatments.